When coating is conducted using an electrodeposition paint, the obtained film sometimes has craters and indentations. One of the causes for this may be, for example, residual rust preventive oil which had been applied on the steel plate substrate. A possible scenario follows: when the film is formed by means of electrodeposition on the steel plate with some rust preventive oil still remaining on it and baking is conducted, residual oil drops and moisture in the electrodeposition paint or from a chemical conversion coating process, come together as a result of heating, and resulting oil drops thus splash and fall on the film surface.
To prevent such a phenomenon, various additives arc known to be added to electrodeposition paint compositions. For example, Japanese unexamined patent publication (Tokkai) Sho 58-25326 and Tokkai Hei 2-4826 discloses process(es) in which a resin containing a polyepoxide and a polyoxyalkylene is added.
However, electrodeposition paint compositions to which conventional additives are added can deteriorate the appearance of the electrodeposition paint compositions because the added resin has a high molecular weight.
The object of this invention is to provide an electrodeposition paint composition which gives a film with fewer craters and indentations, while maintaining performance as an electrodeposition paint.